Growth parameters of the mouse epidermal keratinocyte cell line MKDC4, previously established in this project, were characterized in two growth media: (a) LEP/MK2, containing 0.1 mg/ml bovine serum albumin (BSA) and 0.5% bovine pituitary extract (BPE, equivalent to 0.09 mg protein/ml), and (b) LEP/MK4, containing no BPE and 1.5 mg/ml BSA. The LEP/MK4 medium was found to support colony formation and growth of the MKDC4 cell line, although at a lower rate than the LEP/MK2 medium. Further improvement of the chemically defined LEP/MK4 medium is being planned by use of selected growth factors. The MKDC4 cell line was tested in the presence of inorganic particulates. Its colony forming efficiency and growth rate were not inhibited by the non-toxic ferric oxide, whereas crystalline silica particles (Min-U-Sil quartz) induced a marked dose-dependent inhibition of these parameters.